Cylindrical bonding structure and method of manufacture

ABSTRACT

A cylindrical bonding structure and its method of manufacture. The cylindrical bonding structure is formed over the bonding pad of a silicon chip and the chip is flipped over to connect with a substrate board in the process of forming a flip-chip package. The cylindrical bonding structure mainly includes a conductive cylinder and a solder block. The conductive cylinder is formed over the bonding pad of the silicon chip and the solder block is attached to the upper end of the conductive cylinder. The solder block has a melting point lower than the conductive cylinder. The solder block can be configured into a cylindrical, spherical or hemispherical shape. To fabricate the cylindrical bonding structure, a patterned mask layer having a plurality of openings that correspond in position to the bonding pads on the wafer is formed over a silicon wafer. Conductive material is deposited into the openings to form conductive cylinders and finally a solder block is attached to the end of each conductive cylinder.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority benefit of Taiwanapplication serial No. 91100092, filed Jan. 7, 2002.

BACKGROUND OF THE INVENTION

[0002] 1. Field of Invention

[0003] The present invention relates to a cylindrical bonding structureand its method of manufacture. More particularly, the present inventionrelates to a cylindrical bonding structure for a flip chip package and amethod of fabricating the cylindrical bonding structure.

[0004] 2. Description of Related Art

[0005] In this information-saturated society, working with electronicproducts has become an integral part of our daily life. Currently,integrated circuit products are used for doing business, educating ourchildren or providing us with games for recreation. As a result of rapidprogress in electronic technologies, devices having powerful functionsand personalized designs have been developed. Moreover, most electronicproducts have light and compact design. Nowadays, high-densityintegrated circuits are frequently housed within compact semiconductorpackages such as a flip-chip package and a ball grid array (BGA)package.

[0006] In the flip-chip technique, bumps are formed on the bonding padsof a chip so that the bumps may be attached to corresponding contactpoints on a substrate after flip over. Compared with conventional wirebonding and tape automatic bonding (TAB) packaging techniques, aflip-chip package has the shortest signal transmission path between thechip and the substrate and hence has superior electrical properties. Inaddition, a flip-chip package may be designed to have its back exposedso as to increase heat dissipation rate. Due to the above reasons,flip-chip packaging techniques are widely adopted in the semiconductorfabrication industry.

[0007]FIG. 1A is a partially magnified view showing a connectionconfiguration between a bump on a chip and a contact point on asubstrate in a conventional flip-chip package. A chip 110 normally has aplurality of bonding pads 112 (only one is shown in FIG. 1A). Eachbonding pad 112 has a bump 114. In general, the bump 114 is a solderbump so that the flip-over chip 110 may directly connect with one of thebonding pads 122 (only one is shown in FIG. 1A) on the substrate 120.Since the chip 110 and the substrate 120 each has a differentcoefficient of thermal expansion (CTE), a standoff distance must beprovided between the chip 110 and the substrate 120 so that differentialthermal expansion will not accumulate too much shear stress to break thebumps 114 prematurely.

[0008] Thus, to prevent shear stress from damaging the bumps 114, bumps114 having a great height are often attached to the bonding pads 112 ofthe chip 110 so as to increase the distance of separation between thechip 110 and the substrate 120 as much as possible. However, increasingthe overall height of the bumps 114 must be accompanied by acorresponding increase in outer diameter and volume of the bumps.Moreover, to prevent short-circuiting, pitch between neighboring bumps114 must be increased. Ultimaately, distance between neighboring bondingpads 112 on the chip 110 is hard to reduce.

[0009] In addition, pre-solder material is often applied on the junctionpads 122 of the substrate 120 before the lower end of the bumps 114 areput against the pads 122. In a reflow operation, the low melting pointpre-solder melts and joins the bumps 114 and the junction pads 122together. Because an additional step of applying low melting pointsolder over the junction pads 122 of the substrate 120 has to beconducted, cost of fabricating the substrate 120 is increasedFurthermore, to increase the distance of separation between the chip 110and the substrate 120, high lead solder is a principle ingredient of thebumps 114. Since a high temperature treatment of the bump material toform a spherical shape bump often produces oxide material near thesurface, the bumps 114 and the junction pads 122 often have pooradhesion after the solder reflow process. Poor adhesion often leads tobad electrical connections between the chip and the substrate and a lowoverall yield of the flip chip package.

[0010]FIG. 1B is a partially magnified view showing an alternativeconnective configuration between a bump on a chip and a contact point ona substrate in a conventional flip-chip package. A solder mask 124 isformed over the substrate 120 to pattern out contact area around thejunction pads 122. In fact, there are two major patterning techniquesthat employ the solder mask 124. The first one is called a ‘solder maskdefine’ (SMD) and the other one is called a ‘no solder mask define’(NSMD). In FIG. 1A, a ‘solder mask define’ (SMD) technique is used. Anopening 126 in the solder mask 124 exposes a portion of the junction pad122 so that a bump on the chip 110 is in a corresponding position overthe junction pad 122 on the substrate 120. In FIG. 1B, a ‘no solder maskdefine’ (NSMD) technique is used. An opening 126 in the solder mask 124completely exposes a junction pad 122 so that a bump is completelyconnected to the junction pad 122. The most commonly used material forforming the solder mask 124 is, for example, green lacquer.

[0011] To shorten pitch between neighboring junction pads 122, SMDtechnique such as the one shown in FIG. 1A is often employed. Only aportion of the junction pad 122 is exposed through the solder mask 124for contact with the lower edge of a bump 114 (shown in profile by dashlines 114 a). However, because actual dimension of a bump 114 may varyfrom the standard dimension by ±10%, variation in positional accuracybetween the bump 114 and the junction pad 122 of up to 10 μm ispossible. Furthermore, the opening 126 in the solder mask layer 124 mayhave an intrinsic diametrical variation of about 15 μm. Hence, when thebump 114 and the junction pad 122 are laid on top of each other, thelower edge of the bump 114 may not come into direct contact with thesurface of the junction pad 122. In extreme cases, part of the outeredge of the bump 114 may lean upon the upper corner of the opening 126of the solder mask layer 124 shown by the dash line 114 b in FIG. 1A.Hence, after a solder reflow operation, the bump 114 may not be properlybonded with the junction pad 122 to form a good electrical connection.To ensure proper bonding between the lower edge of the bump 114 with thejunction pad 122, diameter of the opening 126 of a conventional soldermask 124 is generally larger than the external diameter of the bump 114.Since distance between neighboring junction pads 122 must be increasedto accommodate the extension, ultimate level of integration is greatlyreduced.

SUMMARY OF THE INVENTION

[0012] Accordingly, one object of the present invention is to provide acylindrical bonding structure and its method of manufacture capable ofreducing the separation between neighboring bonding pads on a chip whileincreasing distance of separation between the chip and a substrate.Ultimately, reliability of the junctions connecting the chip and thesubstrate is improved and post-packaging life of the chip is extended.

[0013] A second object of this invention is to provide a cylindricalbonding structure and its method of manufacture capable of reducing thediameter of openings on a solder mask for exposing a junction pad sothat distance of separation between neighboring junction pads on thesubstrate is reduced. Consequently, the distance of separation betweenneighboring bonding pads (bumps) on the chip is also reduced.

[0014] A third object of this invention is to provide a cylindricalbonding structure and its method of manufacture that requires noapplication of low melting point solder material on the junction pads ofa substrate or the surface of bumps before conducting a reflow process.Thus, production cost of a flip-chip package is reduced.

[0015] To achieve these and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, theinvention provides a cylindrical bonding structure and its method ofmanufacture. A ball contact metallic layer is formed over the entiresurface of a silicon wafer. A patterned mask layer is formed over theball contact metallic layer. The mask layer has openings that correspondin position to bonding pads on the wafer and expose a portion of theunderlying ball contact metallic layer. By conducting an electroplatingprocess, for example, conductive material is deposited into the openingsof the mask layer to form conductive cylinders. Through electroplatingor printing, solder material is deposited into the openings of the masklayer to form a cylindrical solder cap on the upper surface of theconductive cylinders. The mask layer and the ball contact metallic layeroutside the conductive cylinder are removed. The residual ball contactmetallic layer, the conductive cylinder and the solder cap together forma cylindrical bonding structure. In addition, the cylindrical solder capmay undergo a reflow treatment to transform the cylindrical solder capinto a solder block attached to the upper surface of the conductivecylinder. Alternatively, the deposition of solder material into theopenings may be deleted. After the formation of the conductivecylinders, the mask layer and the ball contact metallic layer outsidethe conductive cylinders are removed. Thereafter, a ball implant processis conducted to attach a solder ball directly onto the exposed surfaceof each conductive cylinder. The residual ball contact metallic contact,the conductive cylinder and the solder ball together form a cylinderbonding structure.

[0016] This invention also provides an alternative cylindrical bondingstructure and its method of manufacture. A ball contact metallic layeris formed over the entire surface of a silicon wafer. A patterned firstmask layer is formed over the ball contact metallic layer. The firstmask layer has openings that correspond in position to bonding pads onthe wafer and expose a portion of the underlying ball contact metalliclayer. By conducting an electroplating process, for example, aconductive material is deposited into the openings of the mask layer toform a conductive cylinder. A patterned second mask layer is formed overthe first mask layer. The second mask layer has openings that expose theupper surface of the conductive cylinders. Similarly, by conductinganother electroplating operation, solder material is deposited into theopenings of the mask layer to form cylindrical solder caps on the uppersurface of all conductive cylinders. The first mask layer, the secondmask layer, and the ball contact metallic layer outside the conductivecylinder are removed. The residual ball contact metallic layer, theconductive cylinder and the cylindrical solder cap together form acylindrical bonding structure. In addition, the cylindrical solder capmay be designed to have an outer diameter smaller than the diameter ofthe opening in the solder mask. Hence, the cylindrical solder cap maypass through the solder mask opening to contact the junction pad on thesubstrate when the chip is flipped over the substrate.

[0017] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary, andare intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments of theinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings,

[0019]FIG. 1A is a partially magnified view showing a connectionconfiguration between a bump on a chip and a contact point on asubstrate in a conventional flip-chip package;

[0020]FIG. 1B is a partially magnified view showing an alternativeconnective configuration between a bump on a chip and a contact point ona substrate in a conventional flip-chip package;

[0021] FIGS. 2A˜2F are schematic cross-sectional views showing theprogression of steps for producing a cylindrical bonding structureaccording to a first embodiment of this invention;

[0022] FIGS. 3A˜3E are schematic cross-sectional views showing theprogression of steps for producing a cylindrical bonding structureaccording to a second embodiment of this invention;

[0023] FIGS. 4A˜4F are schematic cross-sectional views showing theprogression of steps for producing a cylindrical bonding structureaccording to a third embodiment of this invention;

[0024] FIGS. 5A˜5C are schematic cross-sectional views showing anapplication of the third cylindrical bonding structure according to thisinvention to the fabrication of a flip-chip package; and

[0025] FIGS. 6A˜6E are cross-sectional views showing cylindrical bondingstructures fabricated according to this invention with each cylindricalbonding structure having an addition transition layer between theconductive cylinder and the solder cap.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

[0027] FIGS. 2A˜2F are schematic cross-sectional views showing theprogression of steps for producing a cylindrical bonding structureaccording to a first embodiment of this invention. As shown in FIG. 2A,a silicon wafer 210 is provided. Since each chip (not shown) is formedby cutting up the wafer 210 and any redistribution layer or bumps areformed before cutting, the wafer 210 actually represents a congregationof uncut chips. Furthermore, the active surface 212 of the wafer 210 hasa passivation layer 214 and a plurality of bonding pads 216 (only onebonding pad is shown in FIG. 2A). The active surface 212 of the wafer210 refers to the side of the wafer 210 where active devices are formed.The patterned passivation layer 214 exposes the bonding pads 216. Notethat bonding pads 216 includes those original pads on the chips or thebonding pads of any redistribution circuit layer on the chips. Thepurpose of having a redistribution layer is to relocate the originalbonding pads on the chip to some other places on the chip.

[0028] As shown in FIG. 2A, a ball contact metallic layer 220 is formedover the entire active surface 212 (that is, the passivation layer 214)of the wafer 210 by conducting, for example, an electroplatingoperation, an evaporation plating operation or sputtering. The ballcontact metallic layer 220 covers the bonding pads 216 and serves as aninterface between a subsequently formed conductive cylinder 240 (asshown in FIG. 2C) and the bonding pad 216. Hence, the ball contactmetallic layer must be a material that produces as little stress aspossible, has a high adhesive strength, resists corrosion and sticks tosurface quite easily. In general, the ball contact metallic layer is acomposite layer comprising two or more metallic layers, for example,including an adhesive layer and a wetting layer. Common metallicmaterial for fabricating the ball contact metallic layer includestitanium (Ti), tungsten (W), chromium (Cr), copper (Cu), nickel (Ni),cobalt (Co), silver (Ag), gold (Au), tin (Sn), vanadium (V), palladium(Pd) or an alloy of some of the aforementioned metals.

[0029] As shown in FIG. 2B, a patterned mask layer 230 is formed overthe ball contact metallic layer 220. The mask layer 230 has a pluralityof openings 232 (only one is shown in FIG. 2B) that corresponds inposition to the bonding pads 216 and exposes a portion of the ballcontact metallic layer 220. The patterned mask layer 230 is formed, forexample, by forming a photoresist layer over the ball contact metalliclayer 220, conducting a photographic exposure and developing thephotoresist layer. Aside from depositing photoresist and conductingphotographic procedures, the mask layer 230 with openings 232 thereonmay also be formed by a corresponding method using some other materials.

[0030] As shown in FIG. 2C, an electroplating operation is conducted todeposit conductive material into the opening 232 using the ball contactmetallic layer 220 as a seed layer. The opening 232 is partially filledto form a conductive cylinder 240 over the ball contact metallic layer220. The conductive material deposited into the opening 232 is a highmelting point metal or alloy such as tin (Sn), lead (Pb), copper (Cu),gold (Au), silver (Ag), zinc (Zn), bismuth (Bi), magnesium (Mg),antimony (Sb), indium (In) or an alloy containing various combination ofthe aforementioned metals.

[0031] As shown in FIG. 2D, an electroplating operation or a printingoperation is conducted to fill the remaining space of the opening 232with solder material using the conductive cylinder 240 as a seed layer.The solder material forms a cylindrical solder cap 250 over uppersurface of the conductive cylinder 240. Note that the solder material isa metal or alloy having a melting point lower than that of theconductive cylinder. Suitable solder material includes tin (Sn), lead(Pb), copper (Cu) gold (Au), zinc (Zn), bismuth (Bi), magnesium (Mg),antimony (Sb), indium (In) or an alloy containing various combinationsof the aforementioned metals.

[0032] As shown in FIG. 2E, the mask layer 230 and the ball contactmetallic layer 230 outside the conductive cylinder 240 are removed. Thereserved ball contact metallic layer 220, the conductive cylinder 240and the cylindrical solder cap together constitute a cylindrical bondingstructure 260. As an example, the conductive cylinder 240 may containtin and lead in the ratio 5:95 (5Sn/95Pb) or 10:90 (10Sn/90Pb) and thecylindrical solder cap 250 may contain tin and lead in the ratio 63:37(63Sn/37Pb) or 60:40 (60Sn/40Pb). The conductive cylinder 240 can alsobe a copper rod while the cylindrical solder cap 250 can be a tin cap.Alternatively, the conductive cylinder 240 can be a rod made from a highmelting point lead-free alloy such as a tin-silver-copper (Sn/Ag/Cu)alloy and the cylindrical solder cap 250 can be a cap made from a lowmelting point lead-free alloy such as tin-bismuth (Sn/Bi) alloy.

[0033] As shown in FIG. 2F, a reflow treatment is conducted after thecylindrical bonding structure 260 is exposed. In the reflow process, thecylindrical solder cap 250 is partially melted to form a solder block250 a having a hemispherical profile over the upper surface of theconductive cylinder 240.

[0034] As shown in FIGS. 2E and 2F, the cylindrical bonding structure260 of the first embodiment mainly comprises the conductive cylinder 240and the solder block 250 a. The solder block may have a cylindricalshape (the cylindrical solder cap 250 shown in FIG. 2E) or ahemispherical shape (shown in FIG. 2F). The cylindrical bondingstructure 260 serves a similar function as the bump 114 in FIGS. 1A and1B. When the solder block 250 a melts, the conductive cylinder 240 andthe junction pad 122 are joined together. Hence, the cylindrical bondingstructure 260 not only serves as a medium for connecting the chip 110and the substrate 120 together electrically, the conductive cylinder 240also serves as an cushioning pad from the chip 110 that pushes thesolder block further towards the substrate 120. Note that outer diameterof the conductive cylinder 240 is fixed even when height of theconductive cylinder 240 is increased. Hence, distance of separationbetween neighboring cylindrical bonding structures 260 and henceneighboring bonding pads 114 (or bonding pads 216) on the chip 110 isshortened.

[0035] According to the first embodiment, the steps involved infabricating the cylindrical bonding structure include forming a ballcontact metallic layer globally over a wafer and then forming apatterned mask layer over the ball contact metallic layer. The masklayer has an opening that surrounds a bonding pad and exposes a portionof the ball contact metallic layer. An electroplating operation isconducted to partially fill the mask opening with conductive material,thereby forming a conductive cylinder. Another electroplating operationor printing operation is conducted to fill up the remaining space of theopening, thereby forming a cylindrical solder cap on the upper surfaceof the conductive cylinder. Finally, the mask layer and the ball contactmetallic layer outside the conductive cylinder are removed to form thecylindrical bonding structure. Furthermore, a reflow operation may alsobe conducted to transform the cylindrical solder cap into a solder blockhaving a hemispherical shape attached to the upper surface of theconductive cylinder.

[0036] In summary, the method of fabricating the cylindrical bondingstructure according to the first embodiment of this invention mainlyinvolves forming a conductive cylinder over the bonding pad of a chip.The conductive cylinder serves as a conductive medium as well as a padfor cushioning up the distance between the chip and the substrate. Inaddition, by attaching a solder block on the upper end of the conductivecylinder, the conductive cylinder and the junction pad on the substrateare bonded together after the solder block material is partially meltedin a reflow operation. Hence, at the same height level, the conductivecylinder can be designed to have an outer diameter smaller than theouter diameter of a spherical bump in a conventional design. Ultimately,the distance of separation between neighboring cylindrical bondingstructures and hence the corresponding distance of separation betweenneighboring bonding pads on the chip can be reduced.

[0037] FIGS. 3A˜3E are schematic cross-sectional views showing theprogression of steps for producing a cylindrical bonding structureaccording to a second embodiment of this invention. The secondembodiment differs from the first embodiment in that a solder ball isplanted onto the upper surface of the conductive cylinder instead offorming the solder block (or the cylindrical solder cap). Since theinitial steps as shown in FIGS. 3A˜3C for forming the cylindricalbonding structure are identical to the ones shown in FIGS. 2A˜2C,detailed description is omitted.

[0038] As shown in FIG. 3D, the mask layer 330 and the ball contactmetallic layer 320 outside the conductive cylinder 340 are removed toexpose the conductive cylinder 340. As shown in FIG. 3E, a ballplacement operation is conducted to attach a solder ball 350 on theupper surface of the conductive cylinder 340. The solder ball 350 servesan identical function as the solder block 250 a (or cylindrical soldercap 250) in the first embodiment. Hence, a cylindrical bonding structure360 is formed on the bonding pad 314 of the chip 316.

[0039] In summary, one major aspect of both the first and the secondembodiment of this invention is to form a block of solder material ontop of a conductive cylinder for joining the conductive cylinder withjunction pad on the substrate. The block of solder material may beshaped into a variety of forms including cylindrical, spherical orhemispherical The solder block is formed over the conductive cylinder bydepositing solder material into the same opening for forming theconductive cylinder through conducting an electroplating operation orprinting operation as in the first embodiment. Alternatively, a solderball is planted on top of the conductive cylinder as in the secondembodiment.

[0040] FIGS. 4A˜4F are schematic cross-sectional views showing theprogression of steps for producing a cylindrical bonding structureaccording to a third embodiment of this invention. One major aspect inthe third embodiment that differs from the first and the secondembodiment of this invention is the control of outer diameter and lengthof the solder block (or cylindrical solder cap) so that pitch betweenneighboring bonding pads on a chip can be further reduced.

[0041] As shown in FIG. 4A, a wafer 410 having an active surface 412 isprovided. The active surface 412 has a plurality of bonding pads 416thereon. A passivation layer covers the active surface 412 but exposesthe bonding pads 416. A ball contact metallic layer 420 is formed overthe entire active surface 412 (the passivation layer 414) of the wafer410 including the bonding pads 416 by conducting an electroplatingoperation, evaporation plating operation or sputtering, for example.

[0042] As shown in FIG. 4B, a first patterned mask layer 430 is formedover the ball contact metallic layer 420. The first mask layer 430 has aplurality of openings 432 that corresponds in position to the bondingpads 416 and exposes a portion of the ball contact metallic layer 420.Since the patterned first mask layer 430 is formed in a manner similarto the mask layer 230 in the first embodiment, description is notrepeated here.

[0043] As shown in FIG. 4C, an electroplating operation is conducted todeposit conductive material into the openings 432 using the ball contactmetallic layer 420 as a seed layer. Hence, conductive cylinders 440 areformed over the ball contact metallic layer 420. Note that theconductive material is a high melting point metal or alloy.

[0044] As shown in FIG. 4D, a second patterned mask layer 434 is formedover the first mask layer 430. The second mask layer 434 has a pluralityof openings 436 and exposes the central region of the conductivecylinder 440. Since the patterned second mask layer 434 is formed in amanner similar to the mask layer 230 in the first embodiment,description is not repeated here.

[0045] As shown in FIG. 4E, another electroplating operation isconducted to deposit conductive material into the openings 436 using theconductive cylinder 440 as a seeding layer. Hence, a cylindrical soldercap 450 is formed on the upper surface of each conductive cylinder 440.Note that the conductive material deposited into the openings 436 is alow melting point metal or alloy so that the cylindrical solder cap 450has a melting point lower than the conductive cylinder 440.

[0046] As shown in FIG. 4F, the first mask layer 430, the second masklayer 434 and the ball contact metallic layer 420 outside the conductivecylinder 440 are removed. The remaining ball contact metallic layer, theconductive cylinder 440 and the cylindrical solder cap 450 together forma cylindrical bonding structure 460.

[0047] FIGS. 5A˜5C are schematic cross-sectional views showing anapplication of the third cylindrical bonding structure according to thisinvention to the fabrication of a flip-chip package. As shown in FIG.5A, a cylindrical bonding structure 514 according to the thirdembodiment of this invention is formed on the bonding pad 512 of a chip510. The cylindrical bonding structure 514 comprises a ball contactmetallic layer 514 a, a conductive cylinder 514 b and a cylindricalsolder cap 514 c. In addition, a substrate 520 having a solder masklayer 524 and a junction pad 522 thereon is also provided. The soldermask 524 has a plurality of openings 526 that exposes the junction pads522.

[0048] As shown in FIG. 5B, the cylindrical solder cap 514 c has anouter diameter smaller than the diameter of the opening 526 on thesolder mask 524. Hence, tolerance between the cylindrical solder cap 514c on the cylindrical bonding structure 514 and the junction pad 522 onthe substrate 520 is greatly increased. Furthermore, if the conductivecylinder 514 b has an outer diameter greater than the diameter of theopening 526, the cylindrical solder cap 514 c must be designed to have alength greater than the depth of the opening 526. Hence, when thecylindrical solder cap 514 c is lowered into the opening 526, the upperend of the cylindrical solder cap 514 c is able to contact the junctionpad 522.

[0049] As shown in FIG. 5C, a reflow process may be conducted after theupper surface of the cylindrical solder cap 514 c is positioned tocontact the junction pad 522. In the reflow process, the cylindricalsolder cap 514 c partially melts and joins together the conductivecylinder 514 b and the junction pad 522. Moreover, an underfill materialmay be injected into the space between the chip 510 and the substrate520 to protect the cylindrical bonding structure 514 and serve as avibration damper.

[0050] The method of fabricating the cylindrical bonding structureaccording to the third embodiment includes forming a ball contactmetallic layer over the surface of a wafer surface and forming apatterned first mask layer over the ball contact metallic layer. Thefirst mask layer has openings that correspond in position to variousbonding pads on the wafer and exposes a portion of the ball contactmetallic layer An electroplating operation is conducted to depositconductive material into the openings of the first mask layer to formconductive cylinders. A patterned second mask layer is formed over thefirst mask layer. The second mask layer has openings that expose aportion of the upper surface of the conductive cylinders. Similarly,solder material is deposited into the openings of the second mask byconducting an electroplating operation to form cylindrical solder capsover the conductive cylinders. The first mask layer, the second masklayer and the ball contact metallic layer outside the conductivecylinder are removed so that the remaining ball contact metallic layer,the conductive cylinder and the cylindrical solder cap together form acylindrical bonding structure on the chip.

[0051] One major difference between the cylindrical bonding structureaccording to the third embodiment and the first two embodiments is thatthe cylindrical solder cap is designed to have an outer diameter smallerthan opening diameter on the solder mask. Hence, the cylindrical soldercap may easily lower into the opening to contact the junction pad on thesubstrate. This increases the yield of fabricating a flip-chip packageand reduces the diameter of the opening. Ultimately, distance ofseparation between neighboring junction pads on a substrate and distanceof separation between neighboring bonding pads on a chip may both bereduced.

[0052] FIGS. 6A˜6E are cross-sectional views showing cylindrical bondingstructures fabricated according to this invention with each cylindricalbonding structure having an additional transition layer between theconductive cylinder and the solder cap. As shown in FIGS. 6A˜6E, atransition layer 670 is inserted between the conductive cylinder 640 andthe solder block 650 in each case. The transition layer 670 may providedifferent functions according to the constituent materials. Furthermore,the transition layer 670 can be a single layer or a multiple of layers.In FIG. 6A, the transition layer 670 provides a function very similar tothe ball contact metallic layer 620 between the bonding pad 616 and theconductive cylinder 640. The transition layer 670 may contain one or amore layers. The transition layer mainly boosts the connectivity betweenthe conductive cylinder 640 and the solder block 650 or prevents thecollapse of solder block 650 material onto the peripheral section of theconductive cylinder 640 after conducting a reflow operation leading to ashort-circuit between neighboring conductive cylinders.

[0053] The transition layer 670 is fabricated after forming theconductive cylinder 640. The transition layer 670 is formed over theupper surface of the conductive cylinder 640. Thereafter, a cylindricalsolder cap 650 is formed over the transition layer 670 in FIG. 6A, whilea solder block having a hemispherical shape is formed over thetransition layer 670 in FIG. 6B. In FIG. 6C, the transition layer 670 isalso fabricated on the upper surface of the conductive cylinder 640after forming the conductive cylinder 640. However, a solder ball 650 isattached to the transition layer 670 instead of a solder cap. Similarly,in FIGS. 6D and 6E, the transition layer 670 is fabricated on the uppersurface of the conductive cylinder before forming a solder cap over thetransition layer 670. One major difference is that the transition layer670 in FIG. 6D is formed inside the opening of the patterned first masklayer 430 (in FIG. 4C) while the transition layer 670 in FIG. 6E isformed inside the opening of the patterned second mask layer 434 (inFIG. 4D).

[0054] In conclusion, the cylindrical bonding structure according tothis invention is formed by constructing a conductive cylinder over thebonding pad of a chip and using the conductive cylinder to cushion upthe distance of separation between the chip and a substrate. The solderblock on the tip of the conductive cylinder is also used to join theconductive cylinder to a junction pad on the substrate. Compared with aconventional design using spherical bumps, the cylindrical bondingstructure can provide a smaller contact separation. In addition, thesolder block may have a variety of profiles including cylindrical,spherical or hemispherical shape. Note that when the solder block has acylindrical shape, the length and outer diameter of the cylinder may beadjusted to fit into the opening leading to the junction pad.Consequently, outer diameter of the opening may be reduced andseparation between neighboring junction pads may be reduced. In otherwords, separation of neighboring bonding pads on a chip may be reduced.

[0055] Because the conductive cylinder and the junction pad areconnected by partially melting the solder block in a reflow process, thestep of applying a low melting point solder material on the junctionpads of the substrate or the surface of bumps in a conventional designcan be eliminated. Hence, production cost of the flip-chip package isreduced.

[0056] Furthermore, the conventional high-temperature reflow process forshaping the bumps into a spherical shape may result in the formation ofexcessive oxide material on bump surface and may lead to poor bondingbetween the bump and the junction pad. In this invention, however, thesolder block is formed on the upper surface of the conductive cylinder.A high-temperature reflow process for shaping the solder block into aspherical form is not absolutely required. Even if a spherical shape isdemanded, the solder block is shaped using a low-temperature reflowprocess. Hence, not much oxidation occurs at the surface of the solderblock material. Ultimately, a better junction structure is formedlinking up the conductive cylinder and the junction pad.

[0057] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of the presentinvention without departing from the scope or spirit of the invention.In view of the foregoing, it is intended that the present inventioncover modifications and variations of this invention provided they fallwithin the scope of the following claims and their equivalents.

What is claimed is:
 1. A cylindrical bonding structure on a chip havingone or more bonding pads thereon, comprising: a conductive cylinder onthe bonding pad of the chip; and a solder block on the conductivecylinder, wherein the solder block is made from a material having amelting point lower than the conductive cylinder.
 2. The structure ofclaim 1, wherein the bonding pad is the original bonding pad on thechip.
 3. The structure of claim 1, wherein the chip further includes aredistribution circuit layer and the bonding pad is a pad on theredistribution circuit layer.
 4. The structure of claim 1, wherein thesolder block is a solder ball.
 5. The structure of claim 1, wherein thesolder block is a cylindrical solder cap.
 6. The structure of claim 5,wherein the cylindrical solder cap has an outer diameter smaller thanthe conductive cylinder.
 7. The structure of claim 1, wherein materialforming the conductive cylinder is selected from a group consisting oftin, lead, copper, gold, silver, zinc, bismuth, magnesium, antimony,indium and an alloy of the aforementioned metals.
 8. The structure ofclaim 1, wherein material forming the solder block is selected from agroup consisting of tin, lead, copper, gold, silver, zinc, bismuth,magnesium, antimony, indium and an alloy of the aforementioned metals.9. The structure of claim 1, wherein the structure further includes aball contact metallic layer between the conductive cylinder and thebonding pad.
 10. The structure of claim 1, wherein the structure furtherincludes a transition layer between the conductive cylinder and thesolder block.
 11. The structure of claim 10, wherein the transitionlayer has at least one conductive layer.
 12. A method of forming one ormore cylindrical bonding structures over a silicon wafer having anactive surface and at least one bonding pad on the active surface,comprising the steps of: forming a ball contact metallic layer over theentire active surface of the silicon wafer, including the bonding pads;forming a patterned mask layer over the ball contact metallic layer,wherein the first mask layer has at least one opening that correspondsin position to the bonding pad and exposes a portion of the ball contactmetallic layer; depositing conductive material into the opening to forma conductive cylinder over the ball contact metallic layer, wherein theconductive material only partially fills the opening; depositing soldermaterial into the remaining space of the opening to form at least onecylindrical solder cap on the upper surface of the conductive cylinder,wherein the solder material has a melting point lower than theconductive cylinder material; and removing the mask layer and the ballcontact metallic layer outside the conductive cylinder such that theremaining ball contact metallic layer, the conductive cylinder and thecylindrical solder cap together form the cylindrical bonding structure.13. The method of claim 12, wherein the bonding pad is an originalbonding pad on the wafer.
 14. The method of claim 12, wherein the waferfurther includes a redistribution circuit layer and the bonding pad is apad on the redistribution circuit layer.
 15. The method of claim 12,wherein after removing the mask layer and a portion of the ball contactmetallic layer, further includes conducting a reflow operation totransform the cylindrical solder cap into a solder block.
 16. The methodof claim 12, wherein the step of depositing conductive material into theopening includes conducting an electroplating operation.
 17. The methodof claim 12, wherein the step of depositing solder material into theopening includes conducting an electroplating or a printing operation.18. The method of claim 12, wherein after forming the conductivecylinder but before forming the cylindrical solder cap, further includesforming a transition layer over the upper surface of the conductivecylinder so that the cylindrical solder cap is formed over thetransition layer.
 19. The method of claim 18, wherein the transitionlayer has at least one conductive layer.
 20. The method of claim 12,wherein material forming the conductive cylinder is selected from agroup consisting of tin, lead, copper, gold, silver, zinc, bismuth,magnesium, antimony, indium and an alloy of the aforementioned metals.21. The method of claim 12, wherein material forming the solder block isselected from a group consisting of tin, lead, copper, gold, silver,zinc, bismuth, magnesium, antimony, indium and an alloy of theaforementioned metals.
 22. The method of claim 12, wherein materialforming the mask layer includes photoresist.
 23. The method of claim 12,wherein the step of forming the patterned mask layer includes forming aphotoresist layer over the ball contact metallic layer and patterningthe photoresist layer.
 24. The method of claim 23, wherein the step ofpatterning the photoresist layer includes conducting a photo-exposureand developing the exposed photoresist layer.
 25. A method of formingone or more cylindrical bonding structures over a silicon wafer havingan active surface and at least one bonding pad on the active surface,comprising the steps of: forming a ball contact metallic layer over theentire active surface of the silicon wafer, including the bonding pads;forming a patterned mask layer over the ball contact metallic layer,wherein the mask layer has at least one opening that corresponds inposition to the bonding pad and exposes a portion of the ball contactmetallic layer; depositing conductive material into the opening to forma conductive cylinder over the ball contact metallic layer, wherein theconductive material only partially fills the opening; removing the masklayer and the ball contact metallic layer outside the conductivecylinder; and attaching a solder ball onto the upper surface of theconductive cylinder such that the remaining ball contact metallic layer,the conductive cylinder and the solder ball together form thecylindrical bonding structure.
 26. The method of claim 25, wherein thebonding pad is the original bonding pad on the wafer.
 27. The method ofclaim 25, wherein the wafer further has a redistribution circuit layerand the bonding pads are pads on the redistribution circuit layer. 28.The method of claim 25, wherein the step of depositing conductivematerial into the opening includes conducting an electroplatingoperation.
 29. The method of claim 25, wherein after forming theconductive cylinder but before attaching the solder ball, furtherincludes forming a transition layer on the upper surface of theconductive cylinder so that the solder ball is attached to thetransition layer.
 30. The method of claim 29, wherein the transitionlayer has at least one conductive layer.
 31. The method of claim 25,wherein material forming the conductive cylinder is selected from agroup consisting of tin, lead, copper, gold, silver, zinc, bismuth,magnesium, antimony, indium and an alloy of the aforementioned metals.32. The method of claim 25, wherein material forming the solder ball isselected from a group consisting of tin, lead, copper, gold, silver,zinc, bismuth, magnesium, antimony, indium and an alloy of theaforementioned metals.
 33. The method of claim 25, wherein materialforming the mask layer includes photoresist.
 34. The method of claim 25,wherein the step of forming the patterned mask layer includes forming aphotoresist layer over the ball contact metallic layer and patterningthe photoresist layer.
 35. The method of claim 34, wherein the step ofpatterning the photoresist layer includes conducting a photo-exposureand developing the exposed photoresist layer.
 36. A cylindrical bondingstructure on a silicon chip such that the structure may flip over andconnect with a substrate, wherein the chip has at least one bonding padand the substrate has a substrate surface having a patterned solder maskand at least one junction pad thereon, and the solder mask layer has atleast an opening that exposes the junction pad, the cylindrical bondingstructure comprising: a conductive cylinder on the bonding pad of thechip; and a cylindrical solder cap on the conductive cylinder, whereinthe cylindrical solder cap has an outer diameter smaller than thediameter of the opening in the solder mask and a length greater than thedepth of the opening, and the solder material has a melting point lowerthan the conductive cylinder material.
 37. The structure of claim 36,wherein the bonding pad is the original bonding pad on the chip.
 38. Thestructure of claim 36, wherein the chip further includes aredistribution circuit layer and the bonding pad is a pad on theredistribution circuit layer.
 39. The structure of claim 36, whereinmaterial forming the conductive cylinder is selected from a groupconsisting of tin, lead, copper, gold, silver, zinc, bismuth, magnesium,antimony, indium and an alloy of the aforementioned metals.
 40. Thestructure of claim 36, wherein material forming the cylindrical soldercap is selected from a group consisting of tin, lead, copper, gold,silver, zinc, bismuth, magnesium, antimony, indium and an alloy of theaforementioned metals.
 41. The structure of claim 36, wherein thestructure further includes a ball contact metallic layer between theconductive cylinder and the bonding pad.
 42. The structure of claim 36,wherein the structure further includes a transition layer between theconductive cylinder and the cylindrical solder cap.
 43. The structure ofclaim 42, wherein the transition layer has at least one conductivelayer.
 44. A method of forming one or more cylindrical bondingstructures over a silicon wafer having an active surface and at leastone bonding pad on the active surface, comprising the steps of: forminga ball contact metallic layer over the entire active surface of thesilicon wafer, including the bonding pads; forming a patterned firstmask layer over the ball contact metallic layer, wherein the first masklayer has at least one opening that corresponds in position to thebonding pad and exposes a portion of the ball contact metallic layer;depositing conductive material into the opening to form a conductivecylinder over the ball contact metallic layer; forming a patternedsecond mask layer over the first mask layer, wherein the second masklayer has at least one opening than exposes a portion of the conductivecylinder; depositing solder material into the opening to form acylindrical solder cap over the conductive cylinder, wherein the soldermaterial has a melting point lower than the conductive cylindermaterial; and removing the first mask layer, the second mask layer andthe ball contact metallic layer outside the conductive cylinder suchthat the remaining ball contact metallic layer, the conductive cylinderand the cylindrical solder cap together form the cylindrical bondingstructure.
 45. The method of claim 44, wherein the bonding pad is theoriginal bonding pad on the wafer.
 46. The method of claim 44, whereinthe wafer further has a redistribution circuit layer and the bondingpads are pads on the redistribution circuit layer.
 47. The method ofclaim 44, wherein the step of depositing conductive material into thefirst mask layer opening includes conducting an electroplatingoperation.
 48. The method of claim 44, wherein the step of depositingsolder material into the second mask layer opening includes conductingan electroplating operation.
 49. The method of claim 44, wherein afterforming the conductive cylinder but before the cylindrical solder cap,further includes forming a transition layer on the upper surface of theconductive cylinder so that the cylindrical solder cap is formed overthe transition layer.
 50. The method of claim 49, wherein the transitionlayer has at least one conductive layer.
 51. The method of claim 44,wherein material forming the conductive cylinder is selected from agroup consisting of tin, lead, copper, gold, silver, zinc, bismuth,magnesium, antimony, indium and an alloy of the aforementioned metals.52. The method of claim 44, wherein material forming the solder materialis selected from a group consisting of tin, lead, copper, gold, silver,zinc, bismuth, magnesium, antimony, indium and an alloy of theaforementioned metals.
 53. The method of claim 44, wherein materialforming the first mask layer includes photoresist.
 54. The method ofclaim 44, wherein the step of forming the patterned first mask layerincludes forming a photoresist layer over the ball contact metalliclayer and patterning the photoresist layer.
 55. The method of claim 54,wherein the step of patterning the photoresist layer includes conductinga photo-exposure and developing the exposed photoresist layer.
 56. Themethod of claim 44, wherein material forming the second mask layerincludes photoresist.
 57. The method of claim 44, wherein the step offorming the patterned second mask layer includes forming a photoresistlayer over the first mask layer and patterning the photoresist layer.58. The method of claim 57, wherein the step of patterning thephotoresist layer includes conducting a photo-exposure and developingthe exposed photoresist layer.
 59. A method of connecting a chip to asubstrate to form a flip-chip package, wherein the chip has an activesurface having at least a bonding pad thereon, the substrate has asubstrate surface having a patterned solder mask and at least onejunction pad thereon, and the solder mask has at least one opening thatexposes the junction pad, the method comprising the steps of: forming acylindrical bonding structure on the bonding pad of the chip, whereinthe cylindrical bonding structure comprises a conductive cylinder and asolder block, the bottom surface of the conductive cylinder is on top ofthe bonding pad and the bottom surface of the solder block is on theupper surface of the conductive cylinder, and the solder block has amelting point lower than the conductive cylinder; flipping over theactive surface of the chip to face the substrate surface of thesubstrate such that the upper surface of the solder block contacts thejunction pad; and conducting a reflow process to melt the solder blockmaterial so that the conductive cylinder and the junction pad are joinedtogether.
 60. The method of claim 59, wherein the bonding pad is theoriginal bonding pad on the chip.
 61. The method of claim 59, whereinthe wafer further has a redistribution circuit layer and the bondingpads are pads on the redistribution circuit layer.
 62. The method ofclaim 59, wherein material forming the conductive cylinder is selectedfrom a group consisting of tin, lead, copper, gold, silver, zinc,bismuth, magnesium, antimony, indium and an alloy of the aforementionedmetals.
 63. The method of claim 59, wherein material forming the solderblock is selected from a group consisting of tin, lead, copper, gold,silver, zinc, bismuth, magnesium, antimony, indium and an alloy of theaforementioned metals.
 64. The method of claim 59, wherein the solderblock is a cylindrical solder cap.
 65. The method of claim 64, whereinthe cylindrical solder cap has an outer diameter smaller than theopening diameter on the solder mask.
 66. The method of claim 65, whereinthe cylindrical solder cap has a length greater than the depth of theopening on the solder mask.
 67. The method of claim 59, wherein thecylindrical bonding structure further includes a ball contact metalliclayer between the conductive cylinder and the bonding pad on the chip.68. The method of claim 59, wherein the cylindrical bonding structurefurther includes a transition layer between the conductive cylinder andthe solder block.
 69. The method of claim 68, wherein the transitionlayer has at least a conductive layer.